This invention relates to a method and apparatus for testing electric motor rotors, particularly the rotors of induction motors.
Rotors may be described as follows: the rotor is formed from laminated electrical steel punchings, and the rotor winding consists of bars contained in slots punched in the laminations. These bars are short-circuited at both ends by a short-circuiting ring. A bar-end ring structure, without the laminated core, is called a squirrel cage. In small--and medium--horsepower sizes, rotors are made by casting aluminum into the rotor core. In larger sizes of AC motors, cast-aluminum rotors are not practical, and copper bars are inserted into the rotor slots. These copper bars are short-circuited at both ends by a copper end ring, and the end ring is brazed or soldered onto the bars. Sometimes bronze or other alloys are used to replace copper in making the cage and end ring. The size at which the transition between cast-aluminum and copper rotor takes place varies among motor manufacturers, but virtually all rotors in motor sizes of several thousand horsepower and above are built with bar-type rotors.
Rotors are not often tested, but some rotor manufacturing problems can cause poor motor performance. High porosity of an aluminum winding can cause poor current flow, thus limiting torque. Voids in the aluminum winding that develop during the casting process can cause an open circuit at that position on the rotor. Sticking of the aluminum to the steel core can shunt current through the lamination, which reduces motor torque. Other manufacturing processes can create non-uniform air gaps, magnetic imbalance and other problems.
It would be desirable to test each rotor to determine that the completed motor will perform at its rated output, and further, to determine whether there might be a pattern in the performance of a set of rotors indicating a problem in the manufacturing process.